Subwoofer crossover settings: what the numbers actually mean

Spend five minutes in any Australian home-theatre forum and you'll find the same recurring debate: should the crossover be set to 80 Hz, 100 Hz, or "whatever sounds best"? I've lost count of how many systems I've measured where the owner set the crossover by ear, convinced it was dialled in, only to reveal a 12 dB hole right in the male vocal range. The crossover is not a tone control. Set it wrong and it doesn't just affect bass — it degrades the entire frequency response from roughly 60 Hz to 200 Hz, which is some of the most audible real estate in music and film.

So let's go through what the controls actually do, why the defaults are sometimes right and sometimes badly wrong, and how to measure your way to a setting that actually holds up.

What a subwoofer crossover actually does

A crossover is a filter. Specifically, a low-pass filter on the subwoofer (letting bass through, rolling off the highs) and a high-pass filter on the main speakers (letting the mids and highs through, rolling off the lows). The crossover frequency — say, 80 Hz — is the point at which each filter is 3 dB down from its passband level. Not the point where bass suddenly stops, but where it starts rolling off.

That matters because both filters produce a slope, measured in dB per octave. A gentle 12 dB/octave slope means the sub is still contributing meaningfully at 160 Hz; a steep 48 dB/octave slope means it's essentially silent there. The steeper the slope, the less overlap between the two sources — but also the harder it is to time-align them correctly, because steep filters introduce more phase rotation.

Most AV receivers and processors, including the Denon AVR-X3800H (check price), implement the Dolby-mandated 80 Hz crossover at 24 dB/octave (Linkwitz-Riley) for the THX standard. That pairing has a sensible rationale: 80 Hz is low enough that the sub is genuinely non-directional for most listeners, and 24 dB/octave provides enough attenuation that the main speakers aren't expected to reproduce significant bass below their limits. It's a reasonable default. But it's still a default, and rooms don't care about defaults.

Where the crossover frequency should actually sit

The honest answer: it depends on where your main speakers run out of useful bass output. That's not a cop-out — it's genuinely the governing constraint.

A compact standmount like the KEF R3 Meta (check price) is rated to around 45 Hz in-room, but that's with boundary reinforcement doing some heavy lifting. In a real room with the speaker positioned correctly for imaging, you're unlikely to get flat, undistorted output much below 60–70 Hz at realistic listening levels. So setting the crossover at 60 Hz and hoping the speaker handles the rest isn't a plan — it's wishful thinking that will result in driver compression or port noise during loud passages.

The KEF LS50 Meta — a smaller sealed design — rolls off earlier, typically wanting a crossover closer to 100 Hz in most rooms. Full-range floorstanders genuinely producing flat output to 35 Hz can sometimes run with no crossover, though I'd still apply one for subwoofer integration unless you've measured the in-room result carefully.

My practical starting points, before measurement:

• Compact standmounts (≤5-inch woofer): 100–120 Hz
• Larger standmounts, small floorstanders (6–7-inch woofer): 80–100 Hz
• Full-range floorstanders: 60–80 Hz, or bypass if measured flat to 40 Hz

These are starting points for measurement, not endpoints. If you set the crossover and don't then run a sweep with a calibrated microphone, you don't know what you've actually achieved.

Slope steepness and why it's often ignored

Most receivers expose the crossover frequency setting but hide the slope. That's a problem, because two crossovers set to 80 Hz but running at different slopes sound and measure quite differently.

Shallow slopes (12 dB/octave, sometimes called first-order) produce wide overlap zones and are more forgiving of timing misalignment — the gradual transition means a small delay offset doesn't cause a sharp notch. But they leave your main speakers reproducing quite a lot of bass, which can be a problem for small drivers at volume.

Steep slopes (24 or 48 dB/octave) give cleaner handoff between sub and mains but demand tighter alignment. At 48 dB/octave, a timing error of even a few milliseconds can produce a measurable notch at the crossover point. This is why room correction systems like Dirac Live spend real effort on the subwoofer delay setting — and why manually fiddling with the distance setting in your AVR isn't equivalent to a proper time-domain alignment.

I'd generally stick with whatever slope your processor applies by default (usually 24 dB/octave LR4, for good reasons). Overriding it requires knowing what you're doing and measuring the result.

Phase — the most misunderstood control on the subwoofer itself

The phase switch or dial on the back of most subwoofers is not a polarity switch, though it's often treated as one. Flipping polarity (inverting the signal 180°) is a fixed operation. Phase is a frequency-dependent thing, and the "0/180" switch on many subs is a rough approximation that works at some frequencies and not others.

What the phase control is actually trying to compensate for is the timing difference between when the subwoofer produces its output and when the main speakers produce theirs at the crossover frequency. That difference comes from a few sources: the acoustic distance between the sub and your listening position versus the distance from the mains; the filter delay introduced by the crossover itself; and any DSP processing delay in the sub's internal amplifier.

A parametric phase control (which a few subs and most processors provide) lets you align these properly. The blunt 0°/180° switch on a budget subwoofer is a two-position guess. In a room where the answer is somewhere around 90°, neither position is right, and you'll have a partial cancellation at the crossover regardless of which you choose.

The measurement fix is simple: run a nearfield sweep on the sub and a nearfield sweep on the main speaker, then look at where the phase traces diverge at the crossover frequency. If you're using REW (free, available from Room EQ Wizard) you can import both traces and examine the phase alignment directly. A decent USB measurement microphone — the MiniDSP UMIK-1 is the one I reach for most often — costs less than $150 and will tell you more about your crossover alignment than hours of A/B listening.

The 80 Hz Dolby/THX standard: why it's reasonable but not universal

The 80 Hz recommendation in the Dolby and THX standards exists because audio engineers in the 1990s needed a number that would be non-directional in most rooms and achievable by most speakers. Below roughly 80 Hz, human spatial localisation of sound is poor — we don't reliably perceive where the bass is coming from. Above 80 Hz, localisation improves, and the sub's position in the room becomes more audible.

That logic still holds. But the standard was written for cinema screening rooms with professional monitor systems, not for a lounge room in a 1970s triple-fronter in Pascoe Vale where the sub sits in the corner next to the TV cabinet. In real rooms, particularly rooms with significant bass modes (which is most of them), the crossover frequency interacts with room resonances in ways that can amplify or cancel certain frequencies regardless of where you set the dial.

This is why I keep coming back to: set a starting point, then measure. The room always has an opinion, and it's louder than yours.

When to run two crossover points

Bass management in a proper home cinema setup often involves redirecting the LFE channel (the ".1" in 5.1 or 7.1.4) independently from the bass redirected from the main channels. Most processors let you set these separately. The LFE channel is already limited to 120 Hz in the Dolby Digital and DTS specs, so your crossover on LFE just needs to be high enough to pass all that content cleanly — 120 Hz is the standard setting and is nearly always correct.

The crossover applied to the main channels when they're redirected to the sub is the one you need to tune per speaker. Some processors (particularly those with Dirac Live or Audyssey MultEQ XT32) will set this per-channel automatically during calibration. In my experience, Dirac Live does a better job of this than Audyssey — particularly in rooms with difficult bass modes — but both are significantly better than ear-based guessing.

One point I'll make firmly: if your processor runs per-channel crossovers during calibration, don't subsequently override them manually because "80 Hz sounds right". The auto-calibrated values reflect your actual speaker rolloffs and room measurement. Overriding them with a global 80 Hz setting is moving backwards.

The SVS SB-3000 as a reference point for crossover flexibility

The SVS SB-3000 (check price) is a sealed subwoofer I've measured extensively, and it's a useful reference because its internal DSP app gives you genuinely flexible crossover control — variable from 20 to 160 Hz in 1 Hz steps, with slope options and a polarity control. That granularity lets you dial in the crossover precisely once you've done the measurement work, rather than jumping between three preset options.

Most budget subwoofers have far coarser crossover controls, which matters less if you're running the sub through your AVR's bass management (which overrides the sub's own crossover). But if you're using a two-channel integrated without bass management — say, a Marantz Model 40n (check price) feeding the sub via a line-level output — you're relying entirely on the sub's internal crossover, and quality and granularity of that control become much more important.

How to actually verify your crossover is working

The process doesn't require expensive software or professional gear. Download REW. Buy or borrow a calibrated USB microphone. Run a full-range sweep from 20 Hz to 500 Hz with the sub active and mains active simultaneously. Look for three things:

First, a smooth level transition through the crossover region. You want the curve to stay within roughly ±3 dB across the handoff zone — no peaks, no dips deeper than 6 dB. A deep, narrow notch indicates a phase cancellation. A broad suckout suggests the crossover frequency is wrong for these speakers in this room. Second, verify that neither speaker is being pushed below its usable range. If you see the main speaker's level collapsing rapidly below 80 Hz while the sub hasn't fully taken over yet, you need a higher crossover frequency. Third, check for peaks caused by room modes in the 60–120 Hz range. These don't indicate the crossover is wrong — they indicate the room needs treatment or sub EQ, which is a separate problem covered in our guide to subwoofer bass equalisation.

Honestly, the measurement process takes about 40 minutes the first time, including software setup. Most people never do it. Most people also wonder why their system sounds thin on strings or congested on bass-heavy film tracks, and spend money on cables trying to fix something that a free measurement would have diagnosed in ten minutes.

The crossover settings aren't exciting. They don't photograph well. But they're some of the highest-leverage adjustments in the whole system — get them right and your sub disappears into the system the way it's supposed to. Get them wrong and you'll be chasing that slightly "detached" bass quality forever, convinced it must be the sub's fault rather than the 6 dB suckout at 90 Hz hiding in plain sight on the RTA.

— Marcus Vale, Editor